Method and apparatus related to the testing of hydraulic circuits

ABSTRACT

For performing tests on a hydraulic circuit, to the latter there is coupled an auxiliary hydraulic circuit which is associated with a testing stand and which comprises a high-pressure pump of variable output as well as a liquid tank of substantial volume. The rate of liquid flow into and out of said tank is maintained very accurately balanced by a first, continuously operating means effecting a coarse balancing and a second, intermittently operating means effecting a fine balancing. The said second means is actuated when the amplitude of variation in the volume of the liquid contained in said tank exceeds that resulting normally from the temperature changes of the liquid in the entire auxiliary circuit.

United States Patent 1191 Hellouin de Menibus METHOD AND APPARATUSRELATED TO THE TESTING OF HYDRAULIC CIRCUITS [75 Inventor: Olivier AndrH enry 1361's "W Hellouin de Menibus, Etampes, France [73] Assignee: S ociete DAppareillages Et Materiels De Servitudes A.M.S., Choisy-Le-Roi,France 221 Filed: Nov. 4, 1971 21 Appl. No.: 195,603

[52] U.S. Cl. 73/168, 73/40 [51] Int. Cl. G0lm 19/00 [58] Field ofSearch 73/40, 39, 119, 119 A, 73/168, 39, 37

[56] References Cited UNITED STATES PATENTS 2,364,709 12/1944 Greer73/168 2,924,971 2/1960 Schroeder et al.. 73/168 3,270,557 9/1966McClocklin 73/168 3,347,094 10/1967 Schroeder et al 73/168 1 51 July 31,1973 Primary ExaminerLouis J. Capozi Attorney-Edwin E. Greigg [5 7ABSTRACT For performing tests on a hydraulic circuit, to the latterthere is coupled an auxiliary hydraulic circuit which is associated witha testing stand and which comprises a high-pressure pump of variableoutput as well as a liquid tank of substantial volume. The rate ofliquid flow into and out of said tank is maintained very accuratelybalanced by a first, continuously operating means effecting a coarsebalancing and a second, intermittently operating means effecting a finebalancing. The said second means is actuated when the amplitude ofvariation in the volume of the liquid contained in said tank exceedsthat resulting normally from the temperature changes of the liquid inthe entire auxiliary circuit.

8 Claims, 4 Drawing Figures PATENIEU JUL 31 I973 SHEET 1 0F 2 METHOD ANDAPPARATUS RELATED TO THE TESTING OF HYDRAULIC CIRCUITS BACKGROUND OF THEINVENTION This invention relates to a method and a testing stand relatedto the testing and/or inspecting of hydraulic circuits, particularlythose built into airplanes.

The testing of certain hydraulic circuits can be effected only when thepumping means associated with these circuits is in an inoperativecondition. This is the case, for example, when hydraulic circuits ofairplanes are examined on the ground. For this purpose the airplane isplaced on jacks. lt is thus not feasible to have the engines run atoperational speed for several hours. Besides, the costs of such anoperation are very substantial.

The aforenoted problem has been solved by shortcircuiting the pumpintegrated with the circuit tested by means of an auxiliary circuitwhich includes a socalled external pump provided with its own drivingmeans. The auxiliary circuit, the external pump and its driving meansare mounted on a usually mobile chassis or testing stand which isbrought as close as possible to the hose connections provided for theauxiliary circuit. Such an apparatus is sufficient in case there are nostrict conditions imposed on the hydraulic circuit tested, for example,where it is not necessary to replace the tank associated with thecircuit to be tested with another tank for the duration of the tests.

In the majority of hydraulic control circuits, however, particularly incase of hydraulic circuits associated with modern airplanes, thepressure and the volume of the hydraulic liquid enclosed in the tank ofthe circuit must not vary except within predetermined, relatively narrowlimits and further, the liquid has to be degasified. Thus, in such acase, the tank of the hydraulic circuit to be tested comprises a lowpressure stage and a high pressure stage, both entirely closed.

It is apparent that the aforenoted degasification cannot be effectedduring normal operation; such degasification is brought about each timethe circuit is tested. For this reason the auxiliary circuit of thetesting stand includes a relatively large tank in which there prevails apartial vacuum above the level of the liquid. The tank of the circuit tobe tested thus has to be emptied, then the tank of the testing stand hasto be vacuumized, then the tank of the circuit to be tested has to berecharged and the tank of the testing stand disconnected. Only afterthese steps can the testing operation take place. The transfers betweenthe two tanks which sometimes have to be repeated several times are verytime consuming and give rise to a number of problems. One of them asubstantial drawback is the triggering of the safety and alarm systemswhich monitor the pressure and the volume in the low pressure stage ofthe circuit tank.

To avoid the aforenoted disadvantages it has already been proposed toeffect the tests and the degasification simultaneously by maintainingthe tank of the testing stand permanently connected to the circuit. Inorder to ensure that the liquid volume contained in the tank of thecircuit tested remains substantially constant, but since it is notpossible to attach in each case an automatic control means to said tank,it has been necessary to control and to maintain constant the liquidlevel in the tank of the testing stand. The volume of the latter,however, is about times the volume of the tank of the circuit testedwhich represents approximately the maximum liquid quantity delivered perminute. ln order to maintain the volume of the liquid in the tank of thecircuit to be tested about 1/5, it is necessary to maintain the volumecontained in the tank of the testing stand about 1/50. Stateddifferently, the permissible variations must be less than the flowquantity delivered per second. A manual control of a return valve at thetank of the testing stand is thus unsatisfactory; it is necessary thatsuch valve be operated by a means sensing and controlling the liquidlevel in the tank.

The aforenoted solutions represent the present state of the art. Theyleave many problems unresolved due to the untimely transfers of thehydraulic oil between the two tanks which result in a variation of theliquid temperature and/or in a variation of the pressure in the lowpressure stage of the circuit.

It is noted that during the course of the tests as during the course ofnormal operation the temperature of the liquid rises. This change oftemperature is approximately 40 C which corresponds in case of hydraulicoil to a dilatation of fluid of approximately 3%. While a dilatation ofsuch an extent may well be acceptable in the normal operation of thecircuit, it is not admissible in a testing operation. The reason is thatwith the auxiliary circuit the total volume of the fluid is multipliedapproximately by seven or more, and the overall variation of volume dueto the temperaturecaused expansion reaches and even exceeds thepredetermined limit set for the circuit tested. The latter is the onlyone affected by this variation if the lever in the tank of the testingstand is maintained constant.

Furthermore, the conduits of the auxiliary circuit are often ofsubstantial length and cause significant charging losses which are addedto those due to the couplings between the auxiliary circuit and thecircuit to be tested. These charging losses may reach 2 bars at amaximum flow rate when the pressure in the low pressure stage of thehydraulic circuit of an airplane, for example, is about 3 bars. Thus,the flow rate variation which is indispensable to conduct an entireseries of tests may cause sudden and very substantial variations in therelative value of the pressure in the low pressure stage and,consequently, may cause similar variations of volume of the hydraulicliquid enclosed in the tank of the circuit tested. These variations aretoo rapid to permit a correction of the return valve setting by theautomatic valve control to intervene before predetermined limits areexceeded. Stated differently, the delay of response of such automaticvalve control devices is too long.

OBJECT AND SUMMARY OF THE INVENTION It is an object of the invention toprovide an improved hydraulic testing method and system in which thetransfers of inadmissibly disadvantageous effect between the two tanksare eliminated.

For the aforenoted purpose, there is provided a testing stand comprisinga tank of substantial volume wherein the inlet and outlet flow rateswith respect to the tank of the testing stand are automatically and veryaccurately balanced by a continuously operating first means of lesseraccuracy and by a second means of higher accuracy which, however, isactuated only when there is detected such an amplitude of variation inthe liquid volume contained in the tank of the testing stand whichexceeds that resulting normally from the temperature variation of theliquid for the entire circuit associated with the testing stand.Further, the return valve is associated with a means sensing the liquidlevel in the tank and arranged in such a manner that the Feference levelin the said tank varies as a function of the liquid temperature.

Thus, the quasi-constancy of the liquid volume in the low pressure stageof the circuit to be tested is ensured by the equality of the flow ratesof the liquid entering and leaving the tank of the testing stand. Thisequality is first ensured approximately by first means permitting alarge flow rate. This last-named means may be associated, for example,with a device sensing the pressure in the return conduit, or may beformed, in each branch of the testing stand circuit, of a deviceproviding a constant flow rate, whereby the two flows are as close toeach other as possible. The said first means operates continuously; itprevents too rapid variations of liquid volume in the circuit tested butit still allows slow and continuous variations to subsist. In ordertoobtain a near perfect equality of the two flows, the return flow rateis additionally corrected by providing for the return valve an actuatorwhich senses the liquid level in the testing stand tank. For thispurpose, however, the reference lever corresponding to the position ofrest of the level sensor should vary as a function of the liquidtemperature that is, as a function of the total volume of the liquid inthe entire circuit.

The invention will be better understood as well as further objects andadvantages become apparent from the ensuing specification of twoexemplary embodiments taken in conjunction with the drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a schematic illustration of afirst embodiment of the invention;

FIG. 2 is a schematic axial sectional view on an enlarged scale of onecomponent shown in FIG. 1;

FIG. 3 schematically illustrates the principle of liquid level controlby an auxiliary reservoir; and

FIG. 4 is a schematic illustration of a second embodiment of theinvention.

DESCRIPTION OF THE EMBODIMENTS Turning now to FIG. 1, there isschematically shown an auxiliary hydraulic circuit of a testing standused for examining a hydraulic circuit, for example, a hydraulic circuitof an airplane. The auxiliary circuit comprises two pipe couplings l and2 which are, respectively, connected to the high pressure stage of thetank of the circuit to be tested and to the return branch leading to thelow pressure stage of said tank to thus short-circuit the pump of thecircuit tested. The auxiliary circuit includes a tank 3 from which theliquid is driven through a supply conduit 4, the coupling 1 and a nozzle7 to the circuit to be tested. After having passed through the latter,the liquid returns through the coupling 2 and a return conduit 5 intothe tank 3. The displacement of the liquid is effected by a highpressure pump 6 of variable output provided in the supply conduit 4. Thepump 6 is driven by a motor (not shown) mounted on the testing stand. Alevel sensor 8 permits, as it will be described hereinafter, theregulation of the return flow rate as a function of the liquid level inthe tank 3 by controlling a return valve 9 disposed in the conduit 5.The operative connection between the sensor 8 and the valve 9 isindicated with a broken line 10in FIG. 1. The

tank 3 is provided with a vacuum device, not shown, for degasifying theliquid.

The apparatus as described so far is a conventional device. It is to benoted in addition that an auxiliary circuit of the abovedescribed typealso has a certain number of safety devices and control mechanisms suchas filters, exchangers, shut-off valves for the circuit tested,regulating valves, etc., which are not shown and which form no part of,or have no function pertaining to the invention.

The level sensor 8 intermittently controls the return valve 9 forcorrecting the return flow rate in case of slow variations of the latterin order to balance the said return flow rate and the supply flow rate.

In order to permit a complete range of tests of the circuit, it isnecessary to vary the supply flow rate by changing the output of thepump 6. It is noted that these variations may often be quite rapid andfurther, the charge losses in the coupling conduits of the auxiliarycircuit may reach 2 bars at maximum flow rate. It is thus necessary toprovide a valve which varies the return flow as a function of the returnflow pressure and which functions with a variable regulating point(variable desired value). For a small flow rate, the regulating pointshould be substantiallyequal to the operational pressure of the lowpressure stage of the circuit tested, for example, 3 bars, while for themaximum flow rate the regulating point should be equal to thisoperational pressure reduced by the charge losses of the connectingcircuit, thus, for example, 1 bar. For this purpose it is advantageousto dispose in the return circuit an automatically balanced valve whichaffects continuously the return flow rate as .a function of the pressureof the latter and the regulating point of which is a function of thedifference of pressures upstream and downstream of the nozzle 7.

In the embodiment described, the aforenoted automatically balanced valveand the return valve 9 are combined in a sole pneumatic valve havingfour diaphtagms, as illustrated in FIG. 2.

Turning now to FIG. 2, the valve 9 has two stages, a lower stage 9a andan upper stage 9b separated from one another by a rigid septum 13. Theupper stage 9b is divided into four chambers by means of threediaphragms l6, l7 and 18. The chamber bounded by the septum 13 and thediaphragm l8 communicates with, and thus forms part of the returnconduit 5. The liquid enters laterally into the last-named chamber asshown symbolically by the arrow 19 and exits therefrom axially through aport 20, then rejoins the conduit 5 and flows in the direction of thearrow 21 to the tank 3 of the testing stand. The free end of the port 20is formed as a seat for a valve member 22 constituted by the head of apiston 23 attached to the diaphragms l8, l7 and 16. The lower stage 9ais divided into two chambers by a diaphragm 14, to which there isattached a stem 15. The latter traverses the base of the port 20 in afluidtight manner and is adapted to reciprocate linearly along the axisof the port 20. The upper free terminus of the stem 15 constitutes anabutment for the valve member 22 as the latter moves in the direction ofits seat. The conduits 10, 11 and 12 shown in FIG. 1 are illustratedsymbolically in FIG. 2 by arrows bearing the same reference numerals. Inaddition, the arrow 24 indicates that the chamber bounded by thediaphragms 16 and 17 receives a fixed pneumatic signal, while the arrow25 indicates that the chamber defined by the diaphragms 17 and 18 is incommunication with the ambient atmosphere. Thus, the pressure prevailingin the last-named chamber is identical to the atmospheric pressure.

The lower stage 9a functions as a sensor stage responsive to the supplyflow rate. The lower surface of the diaphragm 14 is exposed to thepressure downstream of the nozzle 7 and thus positions the stem 15 todetermine the regulating point of the valve. From this point, as a basevalue, the magnitude of the valve opening is controlled by the pressuredifferential between the return pressure (at 19) and the atmosphericpressure (at 25) and by the pressure differential between the pneumaticsignal transmitted by the level sensor 8 (and indicated at and the fixedpneumatic reference signal (indicated at 24).

The aforedescribed devices permit a balancing of the supply and returnflow rates at each instance and consequently ensure that the liquidvolume contained in the auxiliary circuit is maintained vary accuratelyat a constant value. It is noted, however, that this result is merely anintermediate step to achieve the eventual purpose: i.e., the maintenanceof the liquid volume between two relatively close limits in the circuittested. Such indirect means has been rendered necessary by theunfeasibility to control directly the liquid volume in the circuittested. It is apparent, however, that the constancy of the volumecontained in the auxiliary circuit, that is, the volume in the tank 3,ensures a constancy of the volume contained in the circuit tested onlyif the total volume of liquid is constant. This, however, is not thecase since the liquid temperature increases during the tests. Thisincrease of temperature may attain 40 C, which causes, in case of ahydraulic oil, a volume increase of 3%. Since the auxiliary circuit withits tank 3 which has to be large to effect a sufficient degasification,represents a volume which is about 10 times larger than the volume ofthe circuit tested, this increase of 3% cannot be absorbed by thecircuit tested without exceeding permissible limits. Thus, the volume ofthe liquid contained in the auxiliary circuit should not be maintainedat a constant value, but, on the contrary, it should be varied in such amanner to enable the tank 3 to absorb the variation of volume due to thetemperature changes. This is the reason why the device 8 has to effectregulation of the liquid level in the tank 3 not from a fixed referencelevel, but from a reference level which varies with the temperature ofthe liquid.

For the aforenoted purpose the device 8 is arranged as shown in FIG. 3.It is seen that the device 8 is connected not to the tank 3 but to anauxiliary reservoir 26 communicating with the top and the bottom of thetank 3 by means of flexible conduits not shown. The tank 3 and theauxiliary reservoir 26 are thus communicating vessels in which theabsolute level of the liquid is the same. The reservoir 26 is mountedslidably on a stand and by varying its position thereon, the relativeliquid level in the reservoir is also changed. Thus, if the position ofthe reservoir 26 on its stand is varied as a function of the temperatureof the liquid, the device 8 will control the liquid level with respectto a fixed point on the reservoir 26, that is, with respect to areference level which is a function of the temperature. For this purposethe motion of the auxiliary reservoir 26 is controlled, for example, bya threaded bar 27 which rotates at its lower end in a bearing 28disposed at the terminus of a beam 29 supported on a fulcrum 33. Theother end of the beam 29 is connected by an articulated rod 30 to athermostatic bellows 31 situated in the tank 3 within the liquid. Amanual operator lever 32 associated with the threaded bar 27 permits aninitial setting and also, if need arises, an emptying or filling of thetank of the circuit tested by raising or lowering the auxiliaryreservoir 26 to thus cause, respectively, an opening or a closing of thereturn valve. The quantity of hydraulic oil transferred from the tank ofthe circuit tested to the tank of the testing stand or conversely, is afunction of the height to which the auxiliary reservoir 26 has beenraised or lowered.

It is apparent that for a given volume of the auxiliary circuit and agiven section of the tank 3, the fulcrum 33 may be positioned withrespect to the beam 29 and the pitch of the threaded rod 27 may beselected in such a manner that all variations in the liquid volume duesolely to the variation of temperature are absorbed by the tank 3without any variation of the relative liquid level in the reservoir 26.The balancing of the supply flow rate and the return flow rate, that isthe inlet and outlet flow rates relating to the tank 3 ensure theconstancy of the liquid volume in the circuit tested with the exceptionof the effect of temperature with regard to this volume proper. Theliquid in the circuit tested thus behaves exactly as during the normaloperation of the circuit.

Turning now to FIG. 4, there is shown another embodiment of theinvention. In this embodiment the auxiliary circuit comprises, similarlyto that shown in FIG. 1, couplings l and 2 for the circuit tested, atank 3, a feed conduit 4 containing a high pressure pump 6 of variableoutput, a return conduit 5 and a level sensor and regulator 8.

The device 8, shown symbolically as being operatively connected to thetank 3, is in practice mounted on a movable auxiliary reservoir as shownand described in connection with FIG. 3 in such a manner as to eliminatethe volume variations due to temperature changes. It transmits apneumatic signal forwarded by a line 34 to a pneumatic valve 35contained in a return conduit 36 returning the excess feed flow to thetank 3 as it will be explained hereinafter.

Similarly to the arrangement shown in FIG. I, the only problemencountered is to balance at any moment the feed and return flow rates.A first, coarse balancing is obtained by disposing at the inlet and theoutlet of the tank 3 in the conduits 5 and 4, two pumps 37 and 38 whichhave a low pressure constant output of relatively close value. The pump37 which is disposed in the return conduit 5 at the inlet of the tank 3has an output which generates a flow rate at least equal to the maximumflow rate during operation. The pump' 38 which is disposed in the supplyconduit 4 between the tank 3 and the high pressure pump 6 has a slightlyhigher output, so that the output of the pump 38 is always larger thanthat of the pump 6; the pump 38 thus feeds the circuit tested with aflow rate that is larger than the maximum flow rate in said circuit. Aconduit 39 connects the conduits 4 and 5 at the outlet of the pump 38and at the inlet of the pump 37. In the conduit 39 there is disposed abypass valve 40. The conduit 36, which contains the pneumatic valve 35,is connected to the supply conduit 4 between the pumps 38 and 6. Thedirection of liquid flow in the different branches of the auxiliarycircuit is indicated in FIG. 4 by arrows, the reference characters ofwhich will be considered as representing the corresponding flow ratevalues. Thus, D is the flow rate across pump 37, D is the flow rateacross pump 38, d1 is the flow rate through pump 6, d'l is the returnflow rate across coupling 2, d2 is the flow rate in the conduit 39 andd3 is the flow rate in the con duit 36.

The pump 37 combines the return flow rate through the coupling 2 and thebypass flow rate through the valve 40; thus D 11'] d2.

Similarly, it is apparent that D d'l d2 (13.

When the liquid levels are stabilized in the tank of the circuit testedand in the tank 3 of the testing stand, then d'l d1 and D D d3. Theconduit 36 returns to the tank 3 the liquid resulting from the excessflow rate of the pump 38 with respect to the pump 37.

When the liquid volume in the circuit tested increases, d 1 should beincreased. Since D remains constant, d2 should be reduced. Since D' isconstant and d1 is predetermined, in order to diminish d2, it isnecessary to increase d3. More precisely, the increase of the liquidvolume in the circuit tested brings about a lowering of the level in thetank 3 (or more precisely, in the auxiliary reservoir 26) and theregulator 8 opens to a greater extent the valve 35 which thus causes anincrease of the flow rate d3. Conversely, in case of a decrease of theliquid volume in the circuit tested, a decrease of d'l is obtained by anincrease of d2, that is, by a decrease of d3 which, in turn, resultsfrom a reduction of the flow passage of the valve 35 effected by anincrease in the liquid level in the tank 3.

In case of rapid variations in the flow rate necessitated by the testingprogram and/or the operation of the circuit tested, the sole activecomponents that work in the testing stand are the pump 6 of variableoutput and the bypass valve 40. The sum of the flow rate through thesetwo components equals D, so that dl remains equal to dl.

It is to be understood that a number of modifications may be effected inthe auxiliary circuit without departing from the scope of the invention.In addition to various control and/or safety mechanisms that may beassociated with the circuit as stated earlier, it is feasible to disposethe components differently or to replace them with equivalent means.

What is claimed is:

1. In a'method of testing a hydraulic circuit coupled to an auxiliaryhydraulic circuit of a testing stand, said auxiliary circuit including atank into which hydraulic liquid is introduced and from which hydraulicliquid is withdrawn during the course of liquid circulation in saidcircuits, the improvement comprising the steps of A. continuouslyeffecting a coarse balancing of the flow rates into and out of said tankand B. intermittently effecting a fine balancing of said flow ratesinresponse to those volume variations of the hydraulic liquid in saidtank which are greater than the volume variations for the entireauxiliary circuit due solely to the temperature variations of saidliquid during normal operation.

2. In a testing stand for examining a hydraulic circuit, said testingstand including an auxiliary hydraulic circuit having (a) a tank, (b)first conduit means leading to said tank, (c) second conduit meansleading from said tank, (d) a variable output high-pressure pump in 6said second conduit means, (e) a return valve in said first conduitmeans, (f) a nozzle contained in said second conduit means downstream ofsaid pump, (g) cou- 5 A. first control means associated with at leastone of said conduit means for effecting a continuous coarse balancing ofthe flow rates of hydraulic liquid entering and leaving said tankthrough said first and second conduit means, respectively,

10 B. second control means connected with said return valve to operatethe latter as a function of the liq uid level in said tank related to areference value and C. means connected to said second control means forvarying said reference value as a function of the temperature of saidhydraulic liquid in said auxiliary circuit, said second control meansand said means connected to said second control means effecting anintermittent fine balancing of said flow rates.

3. An improvement as defined in claim 2, said first control means beingformed of a balanced valve disposed in said first conduit means, saidbalanced valve including A. third and fourth conduit means connectingsaid balanced valve with said second conduit means upstream anddownstream of said nozzle,

B. a movable valving member for determining the 30 flow rate ofhydraulic liquid in said first conduit means,

C. a movable abutment limiting the closing movement of said movablevalving member and D. means positioning said movable abutment as a 5function of the pressure difference upstream and downstream of saidnozzle.

4. An improvement as defined in claim 3, said balanced valve beingdisposed in said first conduit means upstream of said return valve.

5. An improvement as defined in claim 3, said return valve and saidbalancing valve being combined into a unitary valve structure includingA. a first diaphragm having one face exposed through said third conduitmeans to the liquid pressure up- 5 stream of said nozzle andanother,'opposite face exposed through said fourth conduit means to theliquid pressure downstream of said nozzle,

B. means affixing said movable abutment to said first diaphragm,

C. a second diaphragm having one face exposed to the liquid pressure insaid first conduit, and another, opposite face exposed to atmosphericpressure,

D. a third diaphragm, having one face exposed to a pressure signaltransmittedby said valve control means as a function of the liquid levelin said tank, and another face exposed to a fixed pressure signal,

E. a fourth diaphragm disposed between said second and third diaphragmsand having one face exposed to said fixed pressure signal and another,opposite face exposed to said atmospheric pressure and F. means affixingsaid second, third and fourth diaphragms to said movable valving member.

6. An improvement as defined in claim 2, said first control meansincluding A. a first additional constant output pump disposed in saidsecond conduit means upstream of said vari- 9 10 able output pump andoperating codirectionally having a reference level, said auxiliaryreservoir t th, being separate from said tank,

3 a Second additional constant Output P p having B. means for varyingsaid reference level as a funca slightly lesser output Said firstadditional tion of the liquid temperature in said auxiliary hypump andbeing disposed in said first conduit 5 draulic circuit h means for drvmghydrauhc hquld towards Said C. means for varying said reference value insaid sectank, f v f h I C a first branch conduit connectin said firstconduit 0nd control means as a uncuon o t e ydnanon m g the referencelevel of the liquid in said auxiliary means at a location upstream ofsaid second additional pump with said second conduit means at areservoir' 8. An im rovement as defined in claim 7 including locationbetween said variable output pump and p Said first additional pump and Alrneans for vertically movably supporting said auxg brgnch g iconnectilng Said i with B r22; i'izzii s i'br maintaining continuouscommuni sai secon con uit means at a ocation etween said variable outputpump and said first additional 9 between tank and Sam aux'lary resetpump, said second branch conduit forming part of said first conduitmeans and containing said return heat expandable means Submerged the yvalve. lic liquid contained in said tank and 7. An improvement asdefined in claim 2, aid m n D. mechanical means for vertically movingsaid auxilconnected to said second control means for varying iaryreservoir in response to the extent of expansaid reference valueincluding sion of said heat expandable means.

A. an auxilary reservoir containing hydraulic liquid

1. In a method of testing a hydraulic circuit coupled to an auxiliaryhydraulic circuit of a testing stand, said auxiliary circuit including atank into which hydraulic liquid is introduced and from which hydraulicliquid is withdrawn during the course of liquid circulation in saidcircuits, the improvement comprising the steps of A. continuouslyeffecting a coarse balancing of the flow rates into and out of said tankand B. intermittently effecting a fine balancing of said flow rates inresponse to those volume variations of the hydraulic liquid in said tankwhich are greater than the volume variations for the entire auxiliarycircuit due solely to the temperature variations of said liquid duringnormal operation.
 2. In a testing stand for examining a hydrauliccircuit, said testing stand including an auxiliary hydraulic circuithaving (a) a tank, (b) first conduit means leading to said tank, (c)second conduit means leading from said tank, (d) a variable outputhigh-pressure pump in said second conduit means, (e) a return valve insaid first conduit means, (f) a nozzle contained in said second conduitmeans downstream of said pump, (g) coupling means provided in said firstand second conduit means for connecting said auxiliary hydraulic circuitto the hydraulic circuit to be tested, the improvement comprising A.first control means associated with at least one of said conduit meansfor effecting a continuous coarse balancing of the flow rates ofhydraulic liquid entering and leaving said tank through said first andsecond conduit means, respectively, B. second control means connectedwith said return valve to operate the latter as a function of the liquidlevel in said tank related to a reference value and C. means connectedto said second control means for varying said reference value as afunction of the temperature of said hydraulic liquid in said auxiliarycircuit, said second control means and said means connected to saidsecond control means effecting an intermittent fine balancing of saidflow rates.
 3. An improvement as defined in claim 2, said first controlmeans being formed of a balanced valve disposed in said first conduitmeans, said balanced valve including A. third and fourth conduit meansconnecting said balanced valve with said second conduit means upstreamand downstream of said nozzle, B. a movable valving member fordetermining the flow rate of hydraulic liquid in said first conduitmeans, C. a movable abutment limiting the closing move-ment of saidmovable valving member and D. means positioning said movable abutment asa function of the pressure difference upstream and downstream of saidnozzle.
 4. An improvement as defined in claim 3, said balanced valvebeing disposed in said first conduit means upstream of said returnvalve.
 5. An improvement as defined in claim 3, said return valve andsaid balancing valve being combined into a unitary valve structureincluding A. a first diaphragm having one face exposed through saidthird conduit means to the liquid pressure upstream of said nozzle andanother, opposite face exposed through said fourth conduit means to theliquid pressure downstream of said nozzle, B. means affixing saidmovable abutment to said first diaphragm, C. a second diaphragm havingone face exposed to the liquid pressure in said first conduit, andanother, opposite face exposed to atmospheric pressure, D. a thirddiaphragm, having one face exposed to a pressure signal transmitted bysaid valve control means as a function of the liquid level in said tank,and another face exposed to a fixed pressure signal, E. a fourthdiaphragm disposed between said second and third diaphragms and havingone face exposed to said fixed pressure signal and another, oppositeface exposed to said atmospheric pressure and F. means affixing saidsecond, third and fourth diaphragms to said movable valving member. 6.An improvement as defined in claim 2, said first control means includingA. a first additional constant output pump disposed in said secondconduit means upstream of said variable output pump and operatingcodirectionally therewith, B. a second additional constant output pumphaving a slightly lesser output than said first additional pump andbeing disposed in said first conduit means for driving hydraulic liquidtowards said tank, C. a first branch conduit connecting said firstconduit means at a location upstream of said second additional pump withsaid second conduit means at a location between said variable outputpump and said first additional pump and D. second branch conduitconnecting said tank with said second conduit means at a locationbetween said variable output pump and said first additional pump, saidsecond branch conduit forming part of said first conduit means andcontaining said return valve.
 7. An improvement as defined in claim 2,said means connected to said second control means for varying saidreference value including A. an auxilary reservoir containing hydraulicliquid having a reference level, said auxiliary reservoir being separatefrom said tank, B. means for varying said reference level as a functionof the liquid temperature in said auxiliary hydraulic circuit and C.means for varying said reference value in said second control means as afunction of the variation in the reference level of the liquid in saidauxiliary reservoir.
 8. An improvement as defined in claim 7, includingA. means for vertically movably supporting said auxiliary reservoir, B.hose means for maintaining continuous communication between said tankand said auxiliary reservoir, C. heat expandable means submerged in thehydraulic liquid contained in said tank and D. mechanical means forvertically moving said auxiliary reservoir in response to the extent ofexpansion of said heat expandable means.